Lincolnshire Police officers attended reports of gatherings in Kenwick Woods just after 8pm on Saturday (February 27).
Officers attended the scene in relation to two gatherings at two separate lodges, and one gathering in the woods.
The groups were dispersed, and 20 people were issued with fixed penalty notices.
Inspector Sarah Constantine said: “This was a blatant breach of Covid regulations.
“Eleven months on and back into a national lockdown, the rules are really clear and so it is frustrating that we still have a small number of completely defiant and irresponsible people who have no regard for the safety of themselves or others.
“While we are still following our 4 E model of engage, explain, encourage and only using enforcement as a last resort, officers are not getting into a debate or discussion with these people about what the rules are or whether they are necessary.
“We’re taking a common sense approach, applying discretion and judgement when engaging with the public. But common sense would dictate that any gatherings of this nature are not acceptable.”